1. Field of the Invention
The present invention relates to a sheet cassette in which recording media such as sheets are stacked, a sheet feeding device including the sheet cassette, and an image forming apparatus.
2. Discussion of the Background
In an image forming apparatus, such as a copier, a printer, a facsimile apparatus and a printing apparatus, an image formed by an image formation part thereof is transferred onto a recording medium such as a recording sheet conveyed from a sheet feeding device, and thereby a record output is obtained.
A sheet cassette in which recording media are stacked is installed in the sheet feeding device, and the sheet feeding device is configured to feed out the recording media in the cassette.
Some sheet cassettes are configured to accommodate recording media of a specific size. In this case, one sheet cassette is necessary for each desired size of recording media, so that the procurement and operating cost of the apparatus is increased. Further, when the image forming apparatus is configured such that one sheet cassette can be installed in the sheet feeding device, when use of a different size of recording media is desired a different sheet cassette must be installed, which is troublesome. Even when the image forming apparatus is configured such that a plurality of sheet cassettes can be installed in the sheet feeding device, often sheet cassettes must be exchanged. Furthermore, because a sheet cassette for recording media of a size that is used infrequently must be also procured, the procurement and operating cost of the apparatus is further increased.
Other sheet cassettes are configured such that recording media of different sizes can be accommodated. In this case, recording media of a desired size is accommodated in the cassette.
Japanese Patent Laid-open publication No. 9-110191 describes a sheet cassette capable of accommodating and feeding out recording media of a different size. The cassette includes a cassette main body and a sliding cassette arranged partially overlapped with the cassette main body to slide relative to the cassette main body, and can be expanded and contracted in the longitudinal direction of recording media by sliding the sliding cassette relative to the cassette main body based on sizes of recording media.
The sheet cassette includes side fences and an end fence that are movable to contact side edges and rear end edges of recording media stacked in the cassette, respectively, to regulate the positions of the side edges and the rear end edges of recording media so that skewing and returning of the recording media occurring when the recording media are fed out of the cassette can be prevented. The end fence is movable to a position corresponding to rear ends of recording media of a certain size. Therefore, when recording media of a smaller size is accommodated in the cassette in the expanded state, depending on the size of the recording media, it occurs that the end fence cannot be moved to the position corresponding to the rear ends of the recording media. In this case, the end fence cannot regulate rear ends of the recording media when the recording media are fed, so that inferior feeding out of recording media may occur.
When a sheet cassette is formed in a box shape in which the upper surface is opened as in the sheet cassette of JP No. 9-110191, it may be conceivable that the cassette is molded using resin. When such a sheet cassette is in the expanded state, torsional deformation easily occurs in the cassette. In particular, when a slit for inserting a guide is formed at a part of the base plate of the cassette as in the sheet cassette of JP No. 9-110191, the rigidity of the cassette is excessively decreased at that part, so that torsional deformation in the base plate of the cassette may not be avoided. If torsional deformation occurs in the base plate of a sheet cassette, the contact condition between the uppermost sheet of recording media stacked in the cassette and a feeding roller arranged to feed the uppermost sheet of the recording media is changed depending upon the size of the recording media stacked in the cassette, so that depending upon the size of recording media stacked in the cassette, inferior feeding of the recording media may occur.